A time domain reflectometry (TDR) level sensing device uses a technique that involves connecting a time domain reflectometer to a conductive element (e.g., a probe, a transmission line, etc.) and immersing the conductive element in a liquid. The time domain reflectometer generates a pulse (i.e., electromagnetic signal) that can exhibit an ultra wide band (UWB) frequency, can be pulse width modulated (PWM), can be formed by an impulse technique, and the like. The pulse generally propagates along and/or proximate the conductive element. A processor in or associated with the TDR level sensing device measures the time taken by the pulse to make a round trip between the reflectometer and the surface of the liquid. In other words, the processor measures a propagation delay from when the pulse leaves the TDR level sensing device to when the pulse returns to the TDR sensing device after having been reflected away from a surface of the liquid.
The TDR level sensing devices are able to provide accurate results since the propagation delay is independent of air temperature, pressure, humidity, and dielectric constant of the liquid. In fact, the precise location of the pulse reflection depends only on the location of the surface of the liquid. The technology behind such fluid level sensing has been referred to as Micropower Impulse Radar (MIR), Guided Wire or Wave Radar (GWR), and Impulse TDR. Some of the basic circuit patterns and methods of employing TDR level sensing are disclosed in U.S. Pat. Nos. 6,644,114, 6,060,915, and 6,055,287 to McEwan.
While the basic TDR level sensing devices of McEwan work well in many circumstances, in some cases the devices experience interference (e.g., ringing, saturation, etc.). If the interference becomes substantial, the reception of the reflected pulse might be compromised and the measurement of the level of the fluid in the tank would be skewed. Therefore, the known TDR level sensing devices were improved by using electrically separate transmitters and receivers, each of which was associated with its own conducting element, along with a float with a coupling device. In other words, bistatic and/or multistatic capabilities were incorporated into the known TDR level sensing devices. These improved TDR level sensing devices and/or systems are disclosed in U.S. Pub. Applns. 2004/0046571 and 2004/0046572 to Champion, et al., and U.S. Pub. Appln. 2004/0059508 to Champion.
Another benefit of the improved TDR level sensing devices was the ability to sense a dielectric mismatch boundary. The dielectric mismatch boundary is a boundary formed between adjacent fluids (e.g., air and gasoline, gasoline and water, and the like) due to the different dielectric constants of each fluid. During operation of the TDR level sensing device, the generated pulses are affected by the dielectric mismatch boundary or boundaries formed in the storage tank. As a result, the TDR level sensing device is able to determine the position of the dielectric mismatch boundary relative to the tank.
Unfortunately, the known TDR or the improved TDR level sensing devices are only able to accurately find the dielectric mismatch boundary between adjacent fluids. The devices were not designed to determine what each of the fluids forming the boundary happen to be. In other words, the TDR level sensing devices can tell a user that a first dielectric mismatch boundary exists, but will not inform the user if there are additional fluid boundaries some distance from the first. This is due to the attenuation of signals in most fluids. The TDR level sensing devices also fail to inform the user what fluids make up the mismatch boundary. As such, the user is unable to determine if, for example, one of the fluids is water.
One known capacitance-based fluid level sensor did have the ability to detect water. However, that sensor was hindered by fluid use restrictions and/or limitations. Specifically, the sensor did not perform as well when used in conjunction with fuels such as, for example, reformulated gasoline and alcohol/gasoline mixtures that exhibit dielectric absorption when measured with capacitance technologies. Because determining whether a fuel is contaminated with water is very desirable, the known capacitance-based fluid level sensor has at least one significant drawback.
Since many users and customers of TDR level sensing devices are interested in testing for water in the storage tank so that the water can be removed, it would be desirable to provide a TDR sensing device with a water sensing element. The invention provides such a device. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.